'Unbelievable victory': President Trump celebrates major hurdle in repealing Obamacare

US President Donald Trump took an early victory lap after his bid to roll back Barack Obama's signature health care law passed a significant hurdle in Congress today.

Mr Trump, who personally prodded and cajoled fellow Republicans into backing a plan that would affect millions of Americans, basked in victory after the vote.

The US president predicted an "unbelievable victory" as he gathered with Republican representatives in the White House Rose Garden, vowing to "finish off" Barack Obama's signature health care law in the Senate, where his own plan faces substantial opposition.

Democrats have vowed to make Republicans pay for backing a plan that they claim will leave millions without care.

Trump boldly promised that that would not be the case.

"Yes, premiums will be coming down. Yes, deductibles will be coming down. But very importantly, it's a great plan," he said.

Groups like the American Medical Association and AARP lobbied against the bill. The AARP's political arm even labelled it an "age tax," arguing that it would result in dramatically higher premiums for seniors.

A number of other groups seized on Jimmy Kimmel's monologue on Monday night, when he talked about the wrenching experience of watching his newborn son go through heart surgery.

Kimmel made the case for covering people with pre-existing conditions, and said that it shouldn't be a matter of wealth as to whether parents are able to cover their children for their care.

"If your baby is going to die and it doesn't have to, it shouldn't matter how much money you make," Kimmel said.

"I think that's something that whether you're a Republican or a Democrat or something else, we all agree on that, right?"

The Obamacare replacement legislation failed to even make it to a floor vote in March, after conservatives and moderates in the Republican caucus baulked.

But in recent weeks, amendments were added in an effort to please the right flank, including a controversial measure that allows states to seek waivers so that insurers could charge those with pre-existing conditions more than healthy customers.

The list of pre-existing conditions is broad and complex, but includes health problems like diabetes, heart problems, cancer and autism.

Victims of sexual assaults will also be considered to have a pre-existing medical condition.

That condition drew criticism from moderate members, but some eventually came to support the legislation after an agreement was struck to add $US8 billion to a fund to "high risk" insurance pools for those who are sick.